The invention relates to a method on a forming section and a forming section.
The task of a forming section is to remove water from fiber suspension fed by the headbox. The consistency of the fiber suspension fed onto the forming section is usually 1% and, after the forming section, the consistency of the web formed on the forming section is, for its part, 18-20%.
When the web is manufactured of watery wood fiber stock, water in the pulp is removed on the forming section through a forming wire or forming wires for starting the formation of the web. Wood pulp fibers remain randomly distributed on the forming wire or between forming wires moving together.
Depending on the grade of the web being manufactured, different types of stocks are used. The amount of water that can be removed from different stocks for achieving a web of good quality is a function of many factors, such as e.g. a function of the desired basis weight of the web, the design speed of the machine, and the desired level of fines, fibers and fill materials in the finished product.
Many types of devices are known on the forming section i.e. former of the web, such as foil strips, suction boxes, hitch rolls, suction rolls, and rolls provided with an open surface, which have been used in many different arrangements and arrays when trying to optimize the amount, time and location of water being removed when forming the web. The manufacture of the web is still partly art and partly science in simply that removing water as quickly as possible does not produce an end-product of best quality. In other words, manufacturing a high-quality end-product especially with great speeds is a function of the amount of dewatering, the dewatering method, the duration of dewatering and the location of dewatering.
When it is desired to maintain or improve the quality of the end-product when transferring to higher production speeds, many times unforeseeable problems are created as the result of which either the production volume has to be decreased for maintaining the desired quality or the desired quality has to be sacrificed for achieving the greater production volume.
A forming section known from prior art is a hybrid former consisting of a single-wire section and a twin-wire section following it, whereby a lower wire forms a second wire of the twin-wire section. The headbox feeds a pulp suspension jet at the beginning of the single-wire section, after which the pulp layer, having received its initial forming on the lower wire, moves onto the twin-wire section in which the formation of the web is continued. On the single-wire section, the web is dewatered only in one direction i.e. through the lower wire and, on the twin-wire section, the web is dewatered in both directions.
The hybrid former can be used in a relatively large basis-weight range, whereby it is possible by means of it to e.g. manufacture fine paper the basis weight of which is in the range of 150-300 g/m2. With a gap former, it is usually not possible to manufacture a web the basis weight of which exceeds the value of 200 g/m2. Thus, there are still a lot of hybrid formers in use and some old fourdrinier-wire formers are modified into hybrid formers.
A problem related to the hybrid former is that the residual variation of the web formed is dependent on the speed of the machine. The upper limit of the speed range of best hybrid formers today is about 1,300 m/min. If the speed of the hybrid former is increased to the value of over 1,300 m/min, also the residual variation of the web formed increases strongly. A web having too large a residual variation is not a saleable product.
FIG. 7 of WO publication 2004/018768 shows a hybrid former according to prior art. The headbox feeds a pulp suspension jet onto a lower wire at the beginning of a single-wire section on top of a breast roll or immediately after it. On the single-wire section, the web is dewatered only in one direction with dewatering devices which can comprise e.g. dewatering strips combined with underpressure or without underpressure, different suction boxes, forming shoes or other equivalents. The single-wire section is followed by a twin-wire section at the beginning of which an upper wire loop forms a gap with the lower wire. Within the upper wire loop, there is a suction box which is divided into three successive compartments in which unequal underpressures can be used. The lower surface of the first compartment of the suction box following the gap of the twin-wire section is constituted of a curvilinear, stationary forming shoe provided with through holes.
FI patent publication 990432 describes a hybrid former in which there is a short single-wire section, which is followed by a twin-wire zone formed between a lower wire loop and an upper wire loop. A slice-lip-adjusted headbox and a breast roll of the fourdrinier wire, which is an open roll, are located so that the headbox feeds a pulp suspension jet at a very small angle onto the fourdrinier wire at the point of the breast roll or in the travel direction of the web after the breast roll. The length of the fourdrinier-wire section is advantageously 0.7-3.0 m. The distance between a vertical plane passing through the central axis of the breast roll and the outmost point of the lip channel of the headbox is 150-250 mm. The height difference between the upper surface of the lower lip of the headbox and the topmost point of the breast roll is 0-10 mm. The impact angle of the headbox jet in relation to the travel direction of the fourdrinier wire is 0-2 degrees.
An article by P. Nyberg and A. Malashenko, “Dilution Control Headbox—Choices, Threats, and Solutions,” published in the Proceedings of the 83rd Annual Meeting, Technical Section, CPPA, Montreal, Canada, January 1997, describes a dilution-adjusted headbox and advantages acquired by it. With a dilution-adjusted headbox, a more effective adjustment of basis weight and fiber orientation are achieved in comparison with a traditional slice-lip-adjusted headbox. The basis weight and the fiber orientation can be adjusted independent of each other and considerably more accurately, whereby variations normally occurring in process parameters can be compensated effectively.
FI patent 116628 describes a forming section of a multi-layer web. FIG. 1 shows a hybrid former in which there is a fourdrinier-wire section and a twin-wire section following it. At the beginning of the fourdrinier-wire section, a first headbox feeds a pulp suspension jet onto the fourdrinier wire and, at the beginning of the twin-wire section, a second headbox feeds a pulp suspension jet on top of a pulp layer travelling on the fourdrinier wire. At the beginning of the fourdrinier-wire section, there is a non-pulsating dewatering zone which consists of a stationary, curvilinear forming shoe at which the pulp suspension jet of the first headbox impacts, advantageously at the angle of 2-6 degrees, in an area immediately after the leading edge of the curvilinear forming shoe. Before the beginning of the twin-wire section, there is a non-pulsating dewatering zone which also consists of a stationary, curvilinear forming shoe at the point of which the pulp suspension jet of the second headbox impacts on the pulp layer travelling on top of the fourdrinier wire. At the beginning of the twin-wire section, there is a non-pulsating dewatering zone consisting of a stationary, curvilinear forming shoe which zone is followed by a pulsating dewatering zone constituted of dewatering strips. In the forming shoes, there is a curvilinear cover provided with holes and possibly underpressure arranged below the cover. In test runs, it has been observed that guiding the lip jet of the headbox with a high speed onto the curvilinear cover constitutes a problem.
FI patent 70739 describes a web-forming unit for manufacturing a paper web. Within a forming wire loop, there are a breast roll and a forming roll. On a section between the breast roll and the forming roll below the forming wire, there are a forming board and a combination of a wet suction box and a wire guiding shoe following it. The headbox feeds a pulp suspension jet on top of the fourdrinier wire on the section following the breast roll. The cover structure of the forming board below the forming wire can be closed, perforated or strip covered. The surface of the forming board is most suitably planar. Dewatering with an open-surfaced forming board takes place most suitably freely, but also a suction effect can be combined with this.
A problem related to arrangements according to prior art is that the formation and the tensile strength ratio of the web are strongly dependent on the jet-wire ratio. An optimum has to be searched for the characteristics of the web in relation to both formation and tensile strength ratio and usually the situation is such that the optima of both factors are not realized with a certain jet-wire ratio. Then one ends up with a compromise in which with higher tensile strength ratios one has to be satisfied with weaker formation.
In the arrangements according to prior art, it is important that the impact point of the lip jet of the headbox can be accurately adjusted to the same point with each run speed. The lip jet impacts in the arrangements according to prior art in the area of the wire in which there are no dewatering strips below the wire, whereby one has to be able to guide the lip jet accurately in the area in question. As the location of the headbox cannot be moved in the machine direction, the location of the impact point of the lip jet is adjusted by adjusting the position of the upper lip of the headbox in the machine direction.